Comparing the two from a user's position, I like Boeshield T9 for the underbody steel because it leaves a paraffin wax residue that may or may not protect longer than CorrosionX. But on the shell the paraffin wax can be an awful looking mess if sprayed on and not (pretty much) wiped completely off.

So I like CorrosionX on the shell and it works very well. I dab it on the panel rivets with a Q-Tip and run a soaked Q-Tip down panel seams, let it soak it good and gently wipe it up. I spray the fittings, hinges, locks with it and wipe them up.

I do this treatment at least quarterly, and probably more if near coastal or northern road salt. It works, going on four years, no corrosion whatsoever on or under the trailer.

Compare this to our 2007 Airstream which sat on the dealers lot with no maintenance for two years before we bought it and it came with filiform corrosion at rivets and seams, and surface rust on underbody steel.